On the summer blot the shadow of the higher stone passes across the lower stone 9m away, allowing the shadow to just reach the top of the lower stone.
The animation shows the shadow’s passage the day before, and exactly on the first possible day of the summer blot:
The gradients of the top of the stones are in good alignment with the sun’s path.
The shadow from the higher the stone ends up in the right height on the lower stone on the first possible day for the summer blot, a half moon cycle after the summer solstice.
Tree branches obscuring the sun which makes the shadow difficult to see.